In addition to its significant use in structural elements such as molded parts, polypropylene has found significant use as a fiber and in yarn, particularly carpet yarn. In order to capitalize on its strength, high melting point and chemical inertness, as well as low cost, the polymer typically used for such applications has been crystalline homopolymer polypropylene. However, this polymer has limited resilience which detracts from its performance in carpeting. Resiliency is a measure of the ability of a fiber or yarn to recover fully its original dimensions upon release of a stress which is compressing it. In the case of polypropylene carpet the poor resiliency is demonstrated by the "walking out" of a sculptured carpet in highly trafficked areas or by the matting which occurs on the walked-on areas of level pile carpets. The matting phenomenon also occurs in upholstery which contains polypropylene pile yarn. Such deficiencies resulted in earlier attempts to improve polypropylene homopolymer performance by modifying the method of crimping the fibers comprising the yarn, U.S. Pat. No. 3,686,848.
Fibers obtained from mechanical blends of homopolymers of polypropylene and polyethylene are known; the thermoshrinkable values of such fibers are good and not very temperature dependent. However, such fibers have the disadvantage of not being very wear-resistant, since they are prone to "fibrillation": the single fiber, after having been subjected to mechanical stress, when examined under a microscope shows longitudinal tears. Such fibrillation is very evident during the manufacture of carpets, and it makes such blends undesirable for this use.
The limited resiliency of polypropylene in carpeting and other fiber/fabric applications is also discussed in "Textile Science and Technology, Polypropylene Fibers-Science and Technology" by M. Ahmed, (Elsevier Press). That reference acknowledges that polypropylene based on commercial fibers is considered intermediate in resilience characteristics between polyester and nylon although "specially prepared fibers" may surpass nylon and approach wool. The reference presents a graph (FIG. 6) that shows resilience, as measured by pile retention, affected by heat setting and draw ratio. It is stated that "(t)here is general agreement that resilient fiber must exhibit high crystalline orientation and high fraction of a-axis oriented crystallites."
While copolymers of propylene with alpha-olefin comonomers have been prepared, such polymers have been used in applications other than yarns, fabrics and carpeting. For example, U.S. Pat. No. 4,322,514 discloses that copolymers based on 80-98 mole % polypropylene, 0.2-15 mole % ethylene and 0.2-15 mole % straight-chained alpha-olefin of C.sub.4 or more result in suitably soft, non- or low-crystalline copolymers having superior transparency, blocking resistance, heat-sealing property and flexibility "for molding into various products; including films, sheets and hollow containers." Blends with other thermoplastic resins such as polypropylene were also recognized for improving the strength, impact resistance, transparency and low-temperature characteristics of the other resin, i.e., to function as a resin modifier. The copolymerization was carried out using an electron donor free catalyst comprising (1) a solid substance containing magnesium and titanium and (2) organometallic compound.
U.S. Pat. No. 4,351,930 discloses a copolymerization process which employs an electron donor containing catalyst for production of a propylene-ethylene-butene-1 copolymer having 80 to 96.5 weight percent propylene, 3 to 17 weight percent ethylene and 0.5 to 5 weight percent butene-1. While a copolymer is produced which contains butene-1, the expressed objective of the process is to provide an improved process for liquid phase ("pool") production of ethylene-propylene copolymers, particularly with enhanced ethylene content and acceptable isotacticity suitable for use as heat sealable films. In passing, it is disclosed that "in addition to the fabrication of film the polymers can be used with advantage in the manufacture of fibers and filaments by extrusion, of rigid articles by injection molding, and of bottles by blow molding techniques." (Essentially a statement of the general uses of thermoplastic polyolefin homopolymers and copolymers).
U.S. Pat. No. 4,181,762 discloses the production of fibers, yarns and fabrics from low modulus polymer. The thermoplastic polymer on which the inventor focuses is an ethylene vinyl acetate (EVA) copolymer, particularly one which has been partially crosslinked to increase the inherently low melting point of EVA. Furthermore, the invention relies on the use of a relatively large diameter fiber in order to achieve a sufficient moment of inertia for that low modulus material to perform satisfactorily in a carpet yarn. While other polymers and copolymers are generally disclosed, they are not defined with any specificity and the copolymers, terpolymers and blends of the present invention are not suggested at all.
U.S. Pat. No. 4,960,820 discloses blends containing "no more than 10% by weight of a low molecular weight, isotactic poly-1-butene polymer with a melt index of greater than 100 to about 1000" with propylene homopolymers and copolymers in order to improve the gloss and clarity of the propylene polymer. The reference includes disclosure of mono- and multifilament fibers with improved stretchability. The reference proposes that such fibers are capable of being spun because "the high melt index butene-1 polymers act as a lubricant or plasticizer for the essentially polypropylene fibers." The reference essentially relates to polypropylene fibers, does not suggest the preparation of yarn and does not even incidentally disclose the use of such fibers for the preparation of carpeting.